Yuhong Qian

A Modulated Hydrothermal (MHT) Approach for the Facile Synthesis of UiO-66-Type MOFs

Developing a general and economically viable approach for the large-scale synthesis of water-stable metal-organic frameworks (MOFs) with repeatable quality remains the key step for their massive production and commercialization. We herein report a green (aqueous solutions), mild (100 °C, 1 atm), and scalable (can be up to kilograms) modulated hydrothermal (MHT) synthesis of UiO-66, an iconic MOF that has been widely studied recently for its high water stability. More importantly, the MHT synthetic approach can be applied to synthesize other water-stable MOFs with structures identical to UiO-66, such as UiO-66-(F)4, UiO-66-(OCH2CH3)2, and UiO-66-(COOH)4, which cannot be obtained via the traditional solvothermal method. Their performance in postcombustion CO2 capture has also been evaluated. Our MHT approach has clearly depicted a roadmap for the facile synthesis of zirconium-based water stable MOFs to facilitate their massive production and commercialization.

Mixed matrix membranes composed of two-dimensional metal–organic framework nanosheets for pre-combustion CO2 capture: a relationship study of filler morphology versus membrane performance

A facile and scalable bottom-up method is used to synthesize a microporous jungle-gym-like metal–organic framework [Cu2(ndc)2(dabco)]n in the morphologies of nanocubes and nanosheets. The obtained MOFs are blended with polybenzimidazole yielding a series of mixed matrix membranes (MMMs), which are evaluated for their performance in pre-combustion CO2 capture (H2/CO2 separation). Pure gas permeation tests indicate that MMMs with partially oriented nanosheet MOFs possess the largest improvement compared with neat polymers, with the overall H2/CO2 separation performance exceeding the 2008 polymer upper bound.

Reversed thermo-switchable molecular sieving membranes composed of two-dimensional metal-organic nanosheets for gas separation

It is highly desirable to reduce the membrane thickness in order to maximize the throughput and break the trade-off limitation for membrane-based gas separation. Two-dimensional membranes composed of atomic-thick graphene or graphene oxide nanosheets have gas transport pathways that are at least three orders of magnitude higher than the membrane thickness, leading to reduced gas permeation flux and impaired separation throughput. Here we present nm-thick molecular sieving membranes composed of porous two-dimensional metal-organic nanosheets. These membranes possess pore openings parallel to gas concentration gradient allowing high gas permeation flux and high selectivity, which are proven by both experiment and molecular dynamics simulation. Furthermore, the gas transport pathways of these membranes exhibit a reversed thermo-switchable feature, which is attributed to the molecular flexibility of the building metal-organic nanosheets.

Synthesis of a Sulfonated Two-Dimensional Covalent Organic Framework as an Efficient Solid Acid Catalyst for Biobased Chemical Conversion

Because of limited framework stability tolerance, de novo synthesis of sulfonated covalent organic frameworks (COFs) remains challenging and unexplored. Herein, a sulfonated two-dimensional crystalline COF, termed TFP-DABA, was synthesized directly from 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid through a previously reported Schiff base condensation reaction, followed by irreversible enol-to-keto tautomerization, which strengthened its structural stability. TFP-DABA is a highly efficient solid acid catalyst for fructose conversion with remarkable yields (97 % for 5-hydroxymethylfurfural and 65 % for 2,5-diformylfuran), good chemoselectivity, and good recyclability. The present study sheds light on the de novo synthesis of sulfonated COFs as novel solid acid catalysts for biobased chemical conversion.

Highly Porous Carbon Derived from MOF-5 as a Support of ORR Electrocatalysts for Fuel Cells

The development of highly competent electrocatalysts for the sluggish oxygen reduction reaction (ORR) at cathodes of proton-exchange membrane fuel cells (PEMFCs) is extremely important for their long-term operation and wide applications. Herein, we present highly efficient ORR electrocatalysts based on Pt/Ni bimetallic nanoparticles dispersed on highly porous carbon obtained via pyrolysis of a metal-organic framework MOF-5. In comparison to the commercial Pt/C (20%), the electrocatalyst Pt-Ni/PC 950 (15:15%) in this study exhibits a pronounced positive shift of 90 mV in Eonset. In addition, it also demonstrates excellent long-term stability and durability during the 500-cycle continue-oxygen-supply (COS) accelerating durability tests (ADTs). The significantly improved activity and stability of Pt-Ni/PC 950 (15:15%) can be attributed to the Pt electron interaction with Ni and carbon support as has been proved in X-ray and microscopic analysis.

Electrocatalysts Derived from Metal–Organic Frameworks for Oxygen Reduction and Evolution Reactions in Aqueous Media

Electrochemical energy conversion and storage devices such as fuel cells and metal-air batteries have been extensively studied in recent decades for their excellent conversion efficiency, high energy capacity, and low environmental impact. However, sluggish kinetics of the oxygen-related reactions at air cathodes, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are still worth improving. Noble metals such as platinum (Pt), iridium (Ir), ruthenium (Ru) and their oxides are considered as the benchmark ORR and OER electrocatalysts, but they are expensive and prone to be poisoned due to the fuel crossover effect, and may suffer from agglomeration and leaching after long-term usage. To mitigate these limits, it is highly desirable to design alternative ORR/OER electrocatalysts with prominent performance. Metal-organic frameworks (MOFs) are a class of porous crystalline materials consisting metal ions/clusters coordinated by organic ligands. Their crystalline structure, tunable pore size and high surface area afford them wide opportunities as catalytic materials. This Review covers MOF-derived ORR/OER catalysts in electrochemical energy conversion, with a focus on the different strategies of material design and preparation, such as composition control and nanostructure fabrication, to improve the activity and durability of MOF-derived electrocatalysts.

Kinetically controlled synthesis of two-dimensional Zr/Hf metal–organic framework nanosheets via a modulated hydrothermal approach

The kinetically controlled synthesis of two-dimensional (2D) metal–organic framework (MOF) nanosheets in the absence of surfactants is rewarding but challenging. We herein describe such a surfactant-free bottom-up synthesis of 2D stable Zr/Hf MOF nanosheets named NUS-8 composed of Zr6O4(OH)4 or Hf6O4(OH)4 clusters and 1,3,5-benzenetribenzoate (BTB3−) via a modulated hydrothermal approach, which allows fast precipitation and stabilization of intermediate 2D metal–organic nanosheets due to the heterogeneous synthetic conditions. Structural analyses based on synchrotron powder X-ray diffraction data confirm the 2D layered structure of NUS-8 with uniform porosity and highly accessible Lewis acid sites suitable for heterogeneous catalysis. 2D NUS-8 nanosheets exhibit excellent stabilities superior to those of their interlocked 3D MOF analogues synthesized from solvothermal synthesis, which are evidenced by comprehensive stability tests. In particular, dynamic mechanical analysis (DMA) experiments suggest that the stability of 2D NUS-8 nanosheets may come from a combination of interlayer shear sliding deformation and out-of-plane tension/compression modes whereas their interlocked 3D architecture is strictly constrained. Because of the alleviated framework strain and accessible active sites, NUS-8 nanosheets exhibit excellent stability and catalytic activity superior to those of their interlocked 3D MOF counterparts. Our work has demonstrated the potential of a modulated hydrothermal approach in the kinetically controlled synthesis of 2D MOF nanosheets, shedding light on future synthesis of 2D hybrid inorganic–organic materials.

Highly efficient photocatalysts by pyrolyzing a Zn–Ti heterometallic metal–organic framework

A series of carbon composite materials containing metal oxides (ZnO and/or TiOx) were synthesized by pyrolyzing a Zn–Ti heterometallic metal–organic framework (MOF) as the precursor. The photocatalytic activities of these composites were evaluated by the photodegradation of methylene blue (MB) in aqueous solutions. The chemical composition and porosity of the prepared composites can be facilely tuned by varying the pyrolysis temperature. The sample being pyrolyzed at 1000 °C exhibited a dramatically increased surface area due to the carbothermal reduction of Zn2+ into metallic Zn followed by vaporization as an extra pore-forming mechanism. Because Zn and Ti are adjacent in the heterometallic secondary building units (SBUs) of the MOF precursor, the catalytically active TiOx sites generated during pyrolysis would remain in the pores and channels formed by Zn vaporization and were readily accessible to MB, leading to a huge increase of photocatalytic activity. Our work reveals a novel mechanism to generate porosity in photocatalysts facilitating the access of substrates to catalytically active sites which can be readily applied to other MOFs containing low boiling point metals.

Fabrication of Highly Stable and Efficient PtCu Alloy Nanoparticles on Highly Porous Carbon for Direct Methanol Fuel Cells

Boosting the durability of Pt nanoparticles by controlling the composition and morphology is extremely important for fuel cells commercialization. We deposit the Pt-Cu alloy nanoparticles over high surface area carbon in different metallic molar ratios and optimize the conditions to achieve desired material. The novel bimetallic electro-catalyst {Pt-Cu/PC-950 (15:15%)} offers exceptional electrocatalytic activity when tested for both oxygen reduction reaction and methanol oxidation reactions. A high mass activity of 0.043 mA/μgPt (based on Pt mass) is recorded for ORR. An outstanding longevity of this electro-catalyst is noticed when compared to 20 wt % Pt loaded either on PC-950 or commercial carbon. The high surface area carbon support offers enhanced activity and prevents the nanoparticles from agglomeration, migration, and dissolution as evident by TEM analysis.

Ultrathin two-dimensional porous organic nanosheets with molecular rotors for chemical sensing

Molecular rotors have played an important role in recent materials chemistry. Although several studies on functional materials containing molecular rotors have been reported for fluorescence sensing, this concept has yet to be realized in two-dimensional (2D) materials. Here we report the preparation of all-carbon, π-conjugated 2D porous organic nanosheets, named NUS-24, which contain flexible tetraphenylethylene (TPE) units as the molecular rotors. NUS-24 nanosheets exhibit high stability, large lateral size, and ultrathin thickness (2–5 nm). The dynamic TPE rotors exposed on the surface of NUS-24 nanosheets can be restricted in the aggregated state with different water fractions, which is reminiscent of the aggregation-induced emission mechanism, thereby leading to the size-selective turn-on fluorescence by volatile organic compounds. Significantly, the ultrathin 2D nanosheets and its composite membranes show much higher sensitivity and selectivity toward Fe3+ ions and nitro-containing compounds sensing, suggesting their potential applications in explosive detection and environmental monitoring.Molecular rotors that fluoresce upon restriction are useful components in functional materials. Here, the authors incorporate molecular rotors into 2D porous organic nanosheets, creating sensitive and selective fluorescent sensors for volatile organic compounds and metal ions.